)GY 





brane. They form a plexiform net-work in the mucous membrane, and are tr en 

 collected into about twenty branches, which pierce the cribriform plate of the eth- 

 moid bone in two groups, a lateral and a medial group, and end in the glomeruli 

 of the olfactory bulb (Fig. 772). Each branch receives tubular sheaths from the 

 dura mater and pia mater, the former being lost in the periosteum of the nose, 

 the latter in the neurolemma of the nerve. 



The olfactory nerves are non-medullated, and consist of axis-cylinders surrounded 

 by nucleated sheaths, in which, however, there are fewer nuclei than are found in 

 the sheaths of ordinary non-medullated nerve fibers. 



The olfactory center in the cortex is generally associated with the rhinencephalon 

 (page 826). 



The olfactary nerves are developed from the cells of the ectoderm which lines 

 the olfactory pits; these cells undergo proliferation and give rise to what are termed 

 the olfactory cells of the nose. The axons of the olfactory cells grow into the over- 

 lying olfactory bulb and form the olfactory nerves. 



THE OPTIC NERVE (N. OPTICUS; SECOND NERVE) (Fig. 773). 



The optic nerve, or nerve of sight, consists mainly of fibers derived from the gan- 

 glionic cells of the retina. These axons terminate in arborizations around the cells 



in the lateral geniculate body, pulvinar, and 

 superior colliculus which constitute the lower 

 or primary visual centers. From the cells of 

 the lateral geniculate body and the pulvinar 

 fibers pass to the cortical visual center, situated 

 in the cuneus and in the neighborhood of the 

 calcarine fissure. A few fibers of the optic nerve, 

 of small caliber, pass from the primary centers 

 to the retina and are supposed to govern 

 chemical changes in the retina and also the 

 movements of some of its elements (pigment 

 cells and cones) . There are also a few fine fibers, 

 afferent fibers, extending from the retina to the 

 brain, that are supposed to be concerned in 

 pupillary reflexes. 



The optic nerve is peculiar in that its fibers 

 and ganglion cells are probably third in the 

 series of neurons from the receptors to the 

 brain. Consequently the optic nerve corre- 

 sponds rather to a tract of fibers within the brain than to the other cranial 

 nerves. Its fibers pass backward and medialward through the orbit and optic 

 foramen to the optic commissure where they partially decussate. The mixed fibers 

 from the two nerves are continued in the optic tracts, the primary visual centers 

 of the brain. 



The orbital portion of the optic nerve is from 20 mm. to 30 mm. in length and has 

 a slightly sinuous course to allow for movements of the eyeball. It is invested by 

 an outer sheath of dura mater and an inner sheath from the arachnoid which are 

 attached to the sclera around the area where the nerve fibers pierce the choroid 

 and sclera of the bulb. A little behind the bulb of the eye the central artery of the 

 retina with its accompanying vein perforates the optic nerve, and runs within it 

 to the retina. As the nerve enters the optic foramen its dural sheath becomes 

 continuous with that lining the orbit and the optic foramen. In the optic foramen 

 the ophthalmic artery lies below and to its outer side. The intercranial portion 

 of the optic nerve is about 10 mm. in length. 



FIG. 773. The left optic nerve and the 

 optic tracts. 



